The refining industry today emphasizes not only the gasoline yield of a hydroconversion process, but also the gasoline quality obtainable by that process, particularly its octane. Gasoline octane is a performance feature of the gasoline which helps prevent engine knocking, and is directly related to the types of hydrocarbon molecules and additives present. As the energy industry implements the federally-mandated elimination of antiknock additives containing lead, it has searched for alternate ways to meet its preferred octane requirements. Since the industry blends gasoline components from several sources in the refinery, such as the reformer, the alkylation plant, the FCC unit, etc., it must optimize the octane contribution from each of these units. It does this by optimizing the production of those components which are high octane. In the United States, the fluid catalytic cracking (FCC) process provides about 35% of the gasoline pool; consequently, refiners are very interested in boosting the octane of the product coming from these type units. Several factors affect the gasoline yield and quality produced by an FCC unit. Feedstock type, catalyst type, and process variables, in particular, temperature and pressure, are among the major factors affecting octane of the gasoline. While refiners can ordinarily optimize the process parameters at which a particular unit operates, they usually have limited flexibility in choosing the feed type. Therefore, changing the catalyst offers refiners a simple and cost-effective way to increase further the gasoline octane of FCC products.
Prior to the advent of zeolites, cracking catalysts consisted primarily of clays, either natural, synthetic, or pretreated, and/or amorphous mixed metal oxides, primarily silica-alumina. When zeolites were introduced, by and large replacing the amorphous catalysts, several beneficial results were readily recognized. For example, zeolite-containing catalysts showed a superior selectivity to gasoline over silica-alumina catalysts. However, this incremental gasoline make tended to come at the expense of C.sub.4 gases, dry gas, and coke. The zeolite-containing catalysts typically reduced the coke yield by about 20%, the H.sub.2, C.sub.1, and C.sub.2 production by 40%, and the C.sub.3 and C.sub.4 olefins. These catalysts also increased the C.sub.5 + gasoline yield, the light cycle oil yield, and the heavy cycle oil yield, while also increasing its density.
Unfortunately, gasolines produced by zeolite-containing catalysts also generally contain more paraffins and aromatics and less olefins than those produced by silica-alumina catalysts, thereby reducing the octane, since reducing olefinicity generally reduces octane. The trend in the refining business continued toward maximizing the gasoline yield at the expense of octane. Toward this end, catalyst manufacturers began developing catalysts containing Y zeolites, primarily hydrogen and rare earth-exchanged Y zeolites, and also replaced active amorphous silica-alumina matrices with less active clay matrices usually bound by a sol comprising silica, alumina, or mixture thereof. This further reduced the olefinicity of the gasoline by reducing and/or eliminating the matrix contribution to the cracking but did further optimize yield.
In general, then, prior art zeolite catalysts show a tremendous activity advantage, and good gasoline selectivity. Unfortunately, they continue to demonstrate higher hydrogen-transfer activity which reduces the olefinic character of this gasoline, thereby reducing the octane rating. Current zeolitic catalysts also minimize the matrix contribution to both the activity and the selectivity. A challenge, therefore, remained: How to modify the catalyst, particularly the zeolite and the matrix chemistry, to achieve both good conversion and good selectivity to gasoline with a high octane rating for vacuum gas oil (VGO). Particularly desirable would be a catalyst which performs well on VGO which contains very few contaminants and for VGO's plus residua and residua blends which contain contaminating metals, especially vanadium. The present invention seeks to provide the answer.